1. Field of the Invention
The present invention relates to a printed wiring board, and more particularly is directed to a printed wiring board having a shielding layer for shielding an electromagnetic wave which may cause an externally induced noise.
2. Description of the Related Arts
Conventionally, an electromagnetic waveshielding layer has been formed on a printed wiring board in order to prevent a malfunction of a wiring circuit caused by an electromagnetic wave from the outside. A printed wiring board having such an electromagnetic wave shielding layer is disclosed in Japanese Examined Utility Model Publication No. 55-29276.
As shown in FIGS. 3 and 4, an electromagnetic wave shielding layer 5 is formed through an insulating layer 3 on the front of the printed wiring circuit which is overlaid on an insulating board 1.
Also, the insulating layer 3, defines a connection land 2 which is provided on a ground circuit 4 in the printed wiring circuit, and the electromagnetic wave shielding layer 5 is electrically connected with the connection land 2 upon the formation of the electromagnetic wave shielding layer 5.
However, when forming the connection land 2 on the ground circuit 4 through the insulating layer 3 in the conventional electromagnetic wave shielding printed wiring board, the connection land 2 has an opening whose contour is not circular but square or polygonal shape which is analogous to that of the ground circuit as shown in FIG. 3. This often causes bubbles to form at the edge of the connection land 2 when forming the electromagnetic wave shielding layer 5, which leads to a problem in the connection reliability.
Also, the connection land 2 as shown in FIGS. 3 and 4 is consequently formed over a wider area, and hence the thickness of the electromagnetic wave shielding layer 5 becomes thinner in that area, which increases a connection resistance, thus leading to a reduction in the shielding effect.
The present invention was therefore conceived in view of the disadvantages in the conventional electromagnetic wave shielding printed wiring board. An objective of the present invention is to provide a printed wiring board which prevents the bubbles from entering the electromagnetic wave shielding layer in the connection land, prevents the thinning of the electromagnetic wave shielding layer on the connection land, and allows the electromagnetic wave shielding printed wiring board to effectively function.